OLED-based displays are currently being considered for use in many applications which presently employ liquid crystal displays (LCDs). OLED-based displays can provide brighter and clearer images than liquid crystal displays and also need less power. However, OLEDs are susceptible to damage resulting from exposure to oxygen and moisture. Such exposure may lead to a reduction in the useful life of the light emitting device. Therefore, hermetic sealing is one of basic requirements for long term performance of OLEDs.
Efforts have been made to hermetically seal OLED-based displays with organic materials, such as epoxy resins. An alternate technology with substantially better performance has been developed by Corning Incorporated, the assignee of this application. In accordance with this approach, a frit-containing paste is made by mixing glass particles, filler particles, e.g., crystalline particles, and a vehicle, e.g., a vehicle comprising one or more solvents and one or more binders and/or dispersing aids. The paste is dispensed on a glass sheet and is sintered using, for example, a high temperature furnace to produce a sintered frit pattern.
The resulting assembly, known as a frit cover glass or simply a cover, is combined with a substrate carrying one or more OLED devices. The cover and the substrate are sealed together by exposing the sintered frit pattern to laser energy. In particular, a laser beam is scanned (traversed) over the sintered frit pattern. The power density of the laser beam and the exposure time are selected so that the temperature of the frit is raised to above its softening point. In this way, the frit adheres to the substrate and forms a strong seal between the cover and substrate. Since the sintered frit is a glass and ceramic material, as opposed to an organic material, the penetration of oxygen and moisture through the frit seal is much slower than through the epoxy seals previously used to encapsulate OLED devices.
Although display devices using sintered frit seals have worked successfully in the past, such devices would benefit from improved bonding strengths, wider processing windows, lower porosities, and/or increased hermeticity. In accordance with the present invention, it has been discovered that the size distribution of the filler particles used in the pastes from which such seals are made, as well as the size distribution of the paste's glass particles, play important roles in determining these properties. As fully detailed below, it has been further discovered that by using filler and/or glass particles having specified distributions at least one and preferably all of the strength, processing window, porosity, and hermeticity parameters can be substantially improved.